Heat resistant and strengthened composite materials and method for producing same

ABSTRACT

Heat resistant and strengthened composite materials are obtained by mixing, and sintering in a non-oxidizing atmosphere, the combination of powders of aluminum nitride and/or silicon nitride, with powders of an oxide of lanthanum, cerium, scandium, yttrium, and/or yttrium aluminum garnet, and with powders or whiskers of silicon carbide, boron nitride and/or carbon. The composite material so produced is characterized by high shock resistance and excellent mechanical strength.

United States Patent [1 1 Komeya et al.

[4 1 Sept. 3, 1974 HEAT RESISTANT AND STRENGTHENED COMPOSITE MATERIALSAND METHOD FOR PRODUCING SAME [73] Assignee: Tokyo Shibaura ElectricC0., Ltd.,

Kawask-shi, Japan [22] Filed: Dec. 23, 1971 [21] Appl. No.: 211,728

[30] Foreign Application Priority Data Dec. 23, 1970 Japan 45-115817[51] Int. Cl C04b 35/52, C04b 35/56, C04b 35/58 [58] Field of Search106/43, 44, 55,65, 56

[56] References Cited UNITED STATES PATENTS 3,108,887 l0/l963 Lenie etal 106/65 Primary ExaminerJames E. Poer Attorney, Agent, or Firm-Oblon,Fisher, Spivak, McClelland & Maier 5 7 ABSTRACT Heat resistant andstrengthened composite materials are obtained by mixing, and sinteringin a nonoxidizing atmosphere, the combination of powders of aluminumnitride and/or silicon nitride, with powders of an oxide of lanthanum,cerium, scandium, yttrium, and/or yttrium aluminum garnet, and withpowders or whiskers of silicon carbide, boron nitride and/or carbon. Thecomposite material so produced is characterized by high shock resistanceand excellent mechanical strength.

4 Claims, N0 Drawings HEAT RESISTANT AND STRENGTHENED COMPOSITEMATERIALS AND METHOD FOR PRODUCING SAME BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to a method for preparingheat resistant and strengthened composite materials having extremelyhigh heat shock resistance.

2. Description of the Prior Art In recent years a significant demand hasdeveloped for high strength materials which are capable of maintainingtheir high strength characteristics even at high temperatures andextremely great heat shocks. Such heat resistant materials are findingapplications in a wide range of fields; from space re-entry vehicles toheat resistant tool jigs.

The present applicants had previously disclosed a heat resistant andhigh strength composite material which was prepared by sintering afinely powdered mixture of aluminum nitride (AlN), at least one oxide ofyttrium oxide (Y O lanthanum oxide (La O cerium oxide (Ce O or scandiumoxide (Sc O and silicon oxide (SiO or trisilicon tetranitride (Si N(Japanese patent application No. 69308 of 1970, No. 62795 of 1970).

Those composite materials not only are characterized by high heatresistance and high strength, but were also more resistant to heat shockthan previously known ceramic materials. Recently, however, a demand hasarisen for materials having even greater heat shock resistance and evengreater mechanical strength, for such applications as are intended tooperate under even more severe conditions of temperature and heat shock.

A need exists, therefore, for a very high heat shock resistant and highstrength composite material.

SUMMARY OF THE INVENTION Accordingly, it is one object of this inventionto pro vide a heat resistant composite material which is characterizedby very high mechanical strength at high temperatures and extremelygreat resistance to heat shocks.

It is another object of this invention to provide a method for preparingheat resistant composite materials which are strengthened by use of aninner fabric structure and without the use of any external fibrousmaterials.

These and other objects have now herein been attained by providing heatresistant and high strength composite material by sintering a mixture ofat least one compound selected from the group consisting of aluminumnitride and silicon nitride, atleast one compound selected from thegroup consisting of oxide of lanthanum, cerium, scandium, yttrium andyttrium aluminum garnet, and at least one compound selected from thegroup consisting of silicon carbide, boron nitride and carbon, in anon-oxidizing atmosphere.

DETAILED DESCRIPTION OF THE INVENTION a powder of an oxide of lanthanum,cerium, scandium yttrium and/or yttrium aluminum garnet (Y Al O Thethird component is a powder of silicon carbide, boron nitride and/orcarbon. If desired, the third component may be in the form of fibers,e.g., whiskers.

The component powders are mixed and suitably dispersed and the mixturepressed to shape. The shaped mixture is then sintered or hot pressedsintered in an nonoxidizing atmosphere.

The third component, i.e., the SiC, BN or C can be used in thiscomposition in amounts of less than 50% by volume and preferably, inamounts of 1% 30% by volume, if it is in the form of a powder and 1%20%, by volume, if it is in the form of whiskers. Greater amounts arenot desirable since, it could adversely affect the sintering step. Themaximum quantity of aluminum nitride or silicon nitride, i.e., the firstcomponent of this composition should be used in amounts of l-97 weightpercent of the total composition.

When the subsequent sintering step is carried out by hot press sinteringthe third component may be used in amounts of 50% by volume and thecomposition will show good resistance to heat shock.

The second component of the present composition is formed from an oxideof lanthanum, cerium, scandium, yttrium and yttrium aluminum garnet, ormixtures thereof. This component may be used in amounts of from 1% 50%by weight. Greater amounts are not desirable since it can decrease themechanical strength of the resulting composite.

Instead of using one of the above oxides, directly, a material which iscapable of producing one of the 0xid'es under the sintering conditionscan also be used. For instance, instead of using yttrium oxide, acompound which is capable of forming yttrium oxide at the sinteringtemperatures such as yttrium carbonyl (Y OC) or yttrium hydroxide (Y(OH)may be used.

Ordinary sintering is carried out at temperatures of l600-2000C if thefirst component is AlN, and l 3001900C if the first component is Si NThe upper limit of the sintering temperature is determined by thesublimation and decomposition temperatures of the first componentnitride and the SiC, BN or C. The lower sintering temperature limit isthe progressive reaction temperature.

Ordinary sintering is preferred for complex shapes because of its lowercost, whereas hot press sintering is desirable where a completely closedobject is intended to be prepared. Hot press sintering temperatures mayrange from 1300 to 2400C at pressures of from 20 to 1500 kg/cm Whenordinary sintering is effected, it is preferred to heat in an inert,non-oxidizing atmosphere, such as in an atmosphere of nitrogen or argon.Sintering can be effected in a single heating step or in multipleheating steps.

The powder components, used-in this invention, may have particle sizesof 0.4-40pt, and the whisker components may have diametersof 05-40;;with lengths of from 40 to p A binder, SllCl'lzBS stearic acid, maybeused'in minor amounts as needed; The quantity of binder used'is notcritical since during the sintering procedure, it will be decomposed orotherwise vaporized out of the system.

Various other heat resistant substances, suchas the heat" resistantoxides, nitrides, carbides, silicides, sulfides, or boron compounds maybe added to the composition to attain specific characteristics. Forinstance, it is desirable to include minor amounts, i.e., l4% andpreferably 2% by volume SiO or A1 in the composition. When the firstcomponent is AlN, it is desirable to use SiO and when the first powdercomponent is Si N it is desirable to use A1 0 in the composition.

The sintered product of this'invention is a highly heat resistant andheat shock resistant material which is characterized by an inner fabricstructure of fibrous crystals within the material, which is composed'offibrous crystals arranged substantially parallel to the surface.

This is a particularly good advantage of this invention as compared withthose prior art heat resistant materials which are composed of surfacefibers.

The three components of this invention can therefore, be summarized asfollows:

the first component AlN, Si N the second component La O Ce O Sc O z s 35 12 SiO can be used with AlN and A1 0 can be used with Si N the thirdcomponent SiC, BN, C (powder or whisker) Having now generally describedthe invention, a further understanding can be obtained by reference tocertain specific examples which are provided herein for purposes ofillustration only and are not intended to be limiting unless otherwisespecified.

EXAMPLE 1 90% by weight of aluminum nitride powder, having an averageparticle size of 1.2 microns, by weight of yttrium oxide powder, havingan average particle size of 1.6 microns, and O, 5, 10, 20, 30, 50% byvolume of silicon carbide powder, having an average particle size of 7microns, or silicon carbide whiskers having a diameter of 10 microns, alength of from 40 to 100 microns, and 5% by weight of stearic acid, as abinder, were mixed. Each mixture was shaped into a large number ofcolumnar pieces, 13 mm in diameter and mm long, at a molding pressure of5000 kg/cm One of the test pieces thus formed was hot pressed in a hotpressing apparatus. Then the test piece was placed into a crucible madeof aluminum nitride, and was packed with aluminum nitride powder. Thetest pieces were heated by the following rates of temperature rise innitrogen gas atmosphere which was prepared by passing the gas throughthe crucible at a rate of 800 liters per hour.

for two hours for four hours for one hour Room temperature to 300C. 300Cto 400C. 400C to 1800C.

cool.

During sintering, the binder was decomposed and evaporated off.

The physical properties of the test pieces sintered are shown in Table1.

TABLE 1 Proportion of raw materials SiC SiC den- Bending heat AlN Y Opowder whisker sity strength shock weight volume (kg/mm) test*** 10* 0**0** 98.4 37.5 less than 10 times times times 90* 10* 20" 0** 92.7 25.5more than times 90* 10* 30** O** 88.2 17.5 more than 100 times 90* 10*50** 0** 85.1 15.2 more than 100 times times 90* 10* 0 10** 93.2 28.4more than 100 times 90* 10* O** 20** 90.5 21.8 more than 100 times 90*10* 0** 30 85.2 13.7 more than 100 times 90* 10* 0** 50** 80.3 10.5 morethan 100 times represents the amount charged as AlNY O system.represents the amount of SiC added to AlN-Y O;, system. represents thenumber of heat shocks before cracks appeared.

As shown in Table 1, if the amount of SiC added increases, the densitydecreases. Therefore, it is preferable that the amount of SiC used beless than 30% volume if it is in the form of a powder, and less than 20%by volume if it is in the form of fibers or whiskers.

These samples were cut and shaped into 10 X 5 mm (low cylinder shape),and were finished by mirror polishing. They were then subjected to aheat shock test by being placed in an electric furnace heated to 1000C(in an atmosphere of air). After 5 minutes they were removed from thefurnace, and cooled rapidly by being dipped into water.

A definite increase in heat shock resistance was noted when thecomposition contained SiN.

The dimensions of the heating chamber, which was of the elevator typeelectric furnace, with a silicon carbide heater, was 300 mm X 300 mm X400 mm.

EXAMPLE 2 Ninety by weight of silicon nitride powder having an averageparticle size of 1.0 micron, 10% by weight of yttrium oxide powder,having an average particle size of 1.6 microns, and 10% by volume ofsilicon carbide, or powder were mixed. The mixture was shaped into alarge number of cylindrical pieces, 13 mm in diameter and 10 mm long,under a molding pressure of 5000 kg/cm One of the test pieces thusformed was hot pressed in a hot pressing apparatus. The test pieces wereheated to a temperature of 1700C in an argon gas atmosphere to effectsintering. The heat shock resistance for these test pieces was measuredas in Example 1. It was found that the SiC containing test pieces hadhigh shock resistance.

TABLEZ EXAMPLE 3-20 Condition of sintering Proportion of raw DensityHeat shock test No. of materials (weight Temp. Method Example (C) (time)3 AlN Y O BN (P) 1800 A 97.5 50 to 100 90 10 4 AIN Y O BN (W) 1800 A95.2 more than 90 10 10 100 5 AIN Y O BN (P) 1800 A 98.1 50 to 100 90 1010 6 AlN Y O CN (P) 1800 A 97.8 more than 90 10 10 100 7 AIN Y O C (W)1800 A 95.5 more than 90 10 10 100 8 AlN YgOg C (W) 1800 A 97.7 morethan 90 10 5 100 9 Al N Y O SiC (P) 1800 B 98.5 more than 90 10 30 10010 AlN Y O SiC (W) 1800 B 98.5 more than 90 10 30 100 11 AlN YgOa SiC(W) 1700 A 96.5 50 to 100 90 20 10 12 AlN Y O SiC (W) 1800 A 98.1 morethan (vol. 13 SEN; YgOg BN (W) 1800 A 95.6 more than 90 10 10 100 14SEN; Y O C (P) 1700 A 94.5 more than 90 10 10 100 15 Sl3N4 Y O C (W)1700 A 97.1 more than 90 10 10 100 16 Si N YgOg SiC (W) 1700 A 96.0 morethan A1 0 2 (vol. 70) 17 Si;,N, Yg03 SiC (W) 1700 B 98.8 more than A1 032 (vol. 18 AlN. Y O SiC (W) 1750 A 97.5 more than SiflN4 10 10 100 (1.1)90 19 Si N La O SiC (W) 1700 A 96.5 more than 90 10 5 100 20 AlN La oSiC (W) 1700 A 97.8 to 100 80 20 5 Control 'Si N Y O 0 1700 A 98.0 lessthan 1 90 10 20 Control AlN 10 0 1750 A 98.5 less than 2 SluN4 90 (P)shown in Table 2 represents Powder". (W) shown in Table 2 representsWhisker".

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The yttrium oxide in the above example can be entirely or partlyreplaced with cerium oxide (Ce O or scandium oxide .(Sc O lanthanumoxide, or yttrium aluminum garnet, with substantially the same results.Namely, by adding SiC, BN or C to the mixture of the above first and thesecond components, good heat shock resistance can be obtained.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade without departing from the spirit or scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A heat resistant composite material formed by sintering a mixture ofpowdered silicon nitride and a sec- 0nd powdered component selected fromthe group consisting of an oxide of lanthanum, cerium, scandium,

yttrium, yttrium aluminum garnet and mixtures thereof, and with a thirdcomponent selected from the group consisting of silicon carbide, boronnitride, carbon and mixtures thereof.

2. The heat resistant material of claim 1 wherein the second componentmaterials are present in an amount of up to 50% by weight of themixture.

3. The heat resistant material of claim 1 wherein the third componentmaterials are used in amounts of less than 50% by volume.

4. The heat resistant material of claim 1 which contains a fibrouscrystalline structure which is substantially parallel to the surface ofsaid material.

2. The heat resistant material of claim 1 wherein the second component materials are present in an amount of up to 50% by weight of the mixture.
 3. The heat resistant material of claim 1 wherein the third component materials are used in amounts of less than 50% by volume.
 4. The heat resistant material of claim 1 which contains a fibrous crystalline structure which is substantially parallel to the surface of said material. 